1. Field of the Invention
The present invention relates to a device and a method for measuring the position of the liquid surface of a melt in a process of pulling a single crystal of semiconductor material by the Czochralski method (hereinafter, abbreviated as the CZ method).
2. Description of the Related Art
A variety of methods are used for growing of single crystals of semiconductor materials. The CZ method is one of the methods. FIG. 6 is a cross-sectional view which schematically shows an apparatus for growing a single crystal according to the CZ method. Referring to FIG. 6, a seed crystal 5a hung on a wire 5c at the end of a seed crystal holder 5b is caused to contact the liquid surface 9 of a melt 2 of single crystal which is contained in a crucible 1. Thereafter, the wire 5c is raised by means of a raising device 5d while the crucible 1 and the raising device 5d are rotated around an axis 5 in opposite directions, and the melt 2 is solidified. Thus, a single crystal 4 is grown into a columnar shape.
To keep the amount of heat, which is applied to the melt 2 with a heater 3, constant in the above-described process, the crucible 1 is raised so that the liquid surface 9 of the melt 2 and the heater 3 are kept in a fixed positional relationship. According to a known technique, the volume of the melt which decreases while a single crystal is being pulled is calculated. The raising amount of the crucible is calculated based on the decrease in volume of the melt and the inside diameter of the crucible. However, the decrease in volume of the melt obtained by the calculation has an error due to the variations in the size of the inside diameter of the crucible, measurement errors, and so forth. In particular, the size of the crucible varies because the crucible is placed in a high temperature environment. Thus, the raising amount of the crucible calculated in the above-described way has an error. As a result, the liquid surface of the melt and the heater can not be kept in a fixed relationship. If the position of the liquid surface of a melt is not kept constant with respect to the heater, heat hysteresis of the grown single crystal will be changed. As a result, crystal defects or the like are generated. Thus, single crystals with sufficient quality cannot be produced.
Heretofore, several methods for measuring the position of the liquid surface of a melt while a single crystal is being pulled have been proposed.
One of the methods is optical trigonometry using a laser beam. According to optical trigonometry, a laser beam is caused to be incident upon the liquid surface of a melt at a predetermined angle. The laser beam reflected from the liquid surface of the melt is detected by means of a detector. However, the liquid surface of the melt fluctuates. This causes the measurement to have an error. Moreover, when the laser beam is reflected from the liquid surface of a melt in the vicinity of a growing single crystal, an error occurs in the measured value of the position of the liquid surface, since the liquid surface of the melt is inclined due to the surface tension with respect to the single crystal.
To eliminate the influence of the fluctuating liquid surface of a melt, according to the method disclosed in Japanese Unexamined Patent Application Publication No. 5-294785, a slit with a small width is set in front of a detector for detecting a laser beam. However, according to this method, the influence of the inclined liquid surface of the melt, which is due to the surface tension with respect to the single crystal, cannot be eliminated. The influence of the inclined liquid surface of the melt becomes greater when the measurement is carried out nearer the single crystal. Therefore, the measurement must be carried out at a position distant from the single crystal. However, in some cases, the measurement is required to be carried out only in the vicinity of the single crystal, depending on the pulling conditions, the structure of the single-crystal-growing apparatus, and so forth. Moreover, the diameter of the single crystal is changed while the single crystal is being pulled. Accordingly, the inclination of the liquid surface is changed. As a result, the measured value of the position of the liquid surface varies. Moreover, it is necessary to provide a furnace with windows for a laser beam on both the projector side and the light-acceptor side of the furnace. Thus, in some cases, it is necessary to modify the structure of the furnace.
Referring to another method, the position of the liquid surface of a melt is measured based on an image of a structure inside a furnace, the image being reflected at the liquid surface of the melt. However, in this case, the above-described influence of the inclined liquid surface cannot be eliminated either. In some cases, the measurement of the liquid surface can be carried out only in the vicinity of the single crystal, depending on the pulling conditions, the structure of the single-crystal-growing apparatus, and so forth. Moreover, the diameter of the single crystal changes as the single crystal is pulled. Accordingly, the inclination of the liquid surface changes. Thus, the measured value varies.
Referring to a method for measuring the position of a liquid surface while eliminating the influence of the inclined liquid surface, it has been proposed that the position of the liquid surface of a melt should be measured based on the central position of the single crystal. Referring to a known method of detecting the central position of a single crystal, the central position is detected based on the position thereof at which a fusion ring formed at the solid-liquid interface between the single crystal and the melt exhibits a maximum diameter (see Japanese Unexamined Patent Application Publication No. 63-238430). According to this method, when the diameter of the single crystal is decreased, the part of the fusion ring which exhibits the maximum diameter is concealed from the grown single crystal. Therefore, when the diameter of the single crystal is decreased, a large error occurs in the measured value. Accordingly, it is necessary that, for determination of the central position of the single crystal, the part of the fusion ring that does not exhibit the maximum diameter should be used. For example, the central position of a single crystal is calculated by approximation of a fusion ring to an ellipse, a circle, or the like in the two-dimensional image thereof according to a least-squares method, a Hough transformation, or the like. However, for the calculation using an approximation, a large number of measurement points are required for higher measurement accuracy. As a result, the amount of calculation increases, and the cost of the measuring device becomes greater.
Japanese Unexamined Patent Application Publication No. 2-102187 proposes a method for measuring the position of a liquid surface without using such an approximation. According to this method, the center of a fusion ring is detected based on the luminance distribution in the horizontal direction of an image. The position of the fusion ring is determined based on the luminance distribution in the vertical direction measured through the detected center of the fusion ring. The distance corresponding to the radius of the single crystal is determined as the peak-to-peak distance in the horizontal direction, based on the position of the fusion ring in the vertical direction. The distance is then corrected. Thus, the position of the center of the single crystal is determined. According to this method, it is required that the side of the fusion ring near a measuring device be entirely observed while the single crystal is being pulled. In some cases, the measurement cannot be carried out, depending on the pulling conditions, the structure of the single crystal growing apparatus, and so forth.